Responsa for Bava Metzia 160:6

Q. A claims that B released his (A's) debtor and sureties without his knowledge or permission. B avers that A had instructed him to release them.
A. If witnesses testify that the debtor owed money to A at the time B released him, B must reimburse A with the amount the debt was worth before the debtor and the sureties were released by B. If there are no witnesses to determine the value of the debt, B must state under oath the extent of the damage he has caused to A, and must compensate him for this damage. But, if A can produce no witnesses to testify that the debtor owed him money, B is free from obligation. We must be satisfied with B's explanation of his act, since he could have denied his act altogether, and since A might have instructed B to release his debtor for the simple reason that the debtor had repaid his debt.
This Responsum is addressed to Rabbi Yedidyah.
SOURCES: Cr. 13; Mord. B. K. 96; Tesh. Maim, to Nezikin, 12. Cf. Agudah B. K. 110; Moses Minz, Responsa 44; ibid. 74a.

Q. A borrowed a book from B for one hour with the understanding that after the hour B would send for it, and if not, he should bring the book back to B. A few hours later a fire broke out in A's house, as a result of which, A was compelled to flee for his life, as the Gentiles were accustomed to throw into the fire Jews in whose houses a fire broke out. A could not, therefore, save B's book.
A. After the period, for which A borrowed the book, had passed, A's responsibility for the book became that of a hired watchman. Therefore, if, before he fled, A could have hired somebody to save the book, and did not do so, he was liable for its loss. But if this was impossible, he was free from obligation.
SOURCES: Pr. 140; Mord. B. M. 376; Agudah B. M. 152.

Q. A lent money to B receiving a pledge as security thereof. He stipulated at the time that he be not held responsible for the pledge in case of loss. Is A responsible if the pledge was lost through his wilful neglect?
A. A is not responsible. Although a creditor is in the category of a paid watchman regarding the pledges he receives as security, he is not even in the category of an unpaid watchman, if he renounces his responsibility thereto; and he is not responsible even though the security be lost through his wilful neglect.
SOURCES: Pr. 229; Mord. B. M. 361; Tesh. Maim. to Mishpatim, 11; Agudah B. M. 127.